Conventionally known image detecting device types are a CCD (Charge Coupled Device) and a CMOS (Complementary Metal Oxide Semiconductor). In such image detecting devices, taking a picture is performed by forming an image of an object on a detecting surface of the image detecting device so as to capture an image, and then the image data is electronically read (i.e., output). Such image detecting devices have enabled significant miniaturization of the structure used to form the image, resulting in significant miniaturization overall.
Especially, module cameras and digital still cameras for image input in portable telephones (simply called digital cameras below) have achieved notable miniaturization. Conventionally, imaging lenses used in these compact imaging devices often have been constructed of a single lens element, placing importance on compactness and portability.
However, while miniaturization of imaging devices has been occurring, an increase in the number of pixels and an increase in image size have also been occurring as the state of the art advances. Therefore, higher optical performance of imaging lenses than needed in the prior art has recently also been required of imaging lenses to be used with state-of-the-art image detecting devices, making it difficult to provide sufficient correction of aberrations when the imaging lens consists of a single lens element.
In order to obtain a sufficiently high optical performance that matches the image capturing performance of state-of-the-art image detecting devices, an imaging lens of a two-piece construction has been disclosed, for example, in Japanese Patent No. 2650309 and in Japanese Patent No. 3007695. However, while the image performance can be enhanced by increasing the number of lenses, the overall length of the imaging lens tends to be larger for an imaging lens having a two-piece construction as compared with a one-piece construction. Moreover, it is difficult to regard the imaging lens described in the above-mentioned Japanese patents as having a sufficiently compact construction. Therefore, when this kind of imaging lens is mounted in an imaging device, compactness/portability may be lost, requiring further improvements. Also, when mounting an imaging lens in an imaging device in general, an optical element such as an infrared blocking filter and a cover glass are often placed between the final surface of the imaging lens and the image detecting device. Therefore, in order to assure that there is sufficient space for these optical elements while achieving compactness of the overall length of the device, it becomes necessary to specify a minimal back focal length so that there is a sufficient back focus.